Ive been running Valvoline 20W50 "Not for Street Use" with a bottle of AC Delco Engine Oil Supplement right from the GM dealer with no trouble as of yet. I have also run 10W30 Not For Street Use but it makes my oil pressure light flicker when its hot. I was told that this oil still has zinc in it and is specialy made for flat tappet camshafts. And I broke it in on Rotella T 10W40 as per Comp Cams. And I always thought Rotella T was for the lawnmower and rototiller.
Hope this helps.